Today the extraction of energy from wind is wholy based on variable speed wind turbines. Fixed-speed wind turbines have only come as far as testing phase and are not yet available for commercial use due to their limited output and the very high wind speed required to operate them. Moreover, as wind speed reduces to a low level, these turbines would consume so much energy from the grid that it could even lead to power grid shutdown.
The patents GB 191916385 dated Apr. 6, 1911 and GB 191028025 dated Aug. 3, 1911 as well as the patent US 2012242084 published on. Sep. 27, 2012 presented the manufacture of frame-supported wind turbine blades whose surface is a thin plate that can be furled or unfurled as well as the thin backside of the blade that allows for effective harnessing of wind energy. However, the patents did not point out how the angle of attack should be distributed along the blade length as well as how to adjust the angle of attack in response to wind speeds below the rated speed.
Given the fact that patent US 2012242084 did describe that each blade is rotated on its longitudinal axis 2 to 10 degrees, it also did not point out any rules for adjusting the attack angle. Employing these inventions in the manufacture of fixed-speed wind turbine blades therefore means the wind turbines falling short of economic and technological indicators since there is no way to control the turbine blade at wind speeds below the rated one. It would be problematic if, for example, the wind turbine is operating at a fixed speed and the angle of attack is not increasing when the wind speed drops below the rated one.
A misleading invention in the field of wind mechanics was in 1919 when Albert Betz published his equation for calculating the power of bladed wind turbine as: PTB=1/2ρA0v3Cp where ρ is air density; Ao is the swept area of the blade; v is wind speed and; Cp is the power coefficient. The equation has led wind turbine technology in the direction of increasing the swept area Ao of the blade (i.e. increasing the blade length), while overlooking other important physical factors of the blade such as the angle of attack, blade area and shape as well as the rotational speed of the wind turbine. Consequently, wind turbine blades have been broadly viewed as objects flying in the wind, which leads to the fact that turbine blades are all designed with an aerodynamic airfoil that supports movement of the blades. The author of this invention has come to realize that the equation was due to Albert Betz's application of Newton's second law in calculating the force of wind turbine acting on the wind: F=ma, where m is the mass of the wind and a is the acceleration of the wind. Actually, it is impossible to exert a force on an air mass, and more importantly, Newton's second law is of particle dynamics. Document related to this discovery has been publicly shared on Youtube at: https://Youtu.be/HTaAJQPkrp0 in both English and Vietnamese language.
Bladed wind turbines manufactured based on Betz' law have their blades with an aerodynamic airfoil. U.S. Pat. No. 4,339,230 dated Jul. 13, 1982 detailed a blade design that has both an airfoil-shaped upper surface and an airfoil-shaped lower surface, and again there are neither rules for the angle of attack nor indication of whether the blade width had to reduce towards the blade tip to avoid reduction in power. Moreover, as the rotational speed of the wind turbine depends on wind speed, it is impractical to produce a fixed-speed wind turbine whose energy generated can be fed directly to the grid.
These issues have put a limit on the output of wind turbines and resulted in very high costs of wind turbine manufacture and wind power generation, driving wind power away from becoming a major power source for human.
The correct standpoint would then be the view of wind turbine blade as an obstruction of the airflow, and the blade surface of a very short blade segment should be considered flat. Besides, a truss system should be employed to enhance the strength of the blade and minimize impacts on the wind. In fact, a wind turbine blade is subjected to two kinds of wind forces, one of which is caused by the collision of moving air particles with the blade surface and the other by the pressure drop on the backside of the blade. These forces are resolved into two components, one parallel to the turbine shaft which does not rotate the turbine but blows it down and the other tangential to the rotation orbit of blade segments which is useful, as it produces work to rotate the turbine. The author of this invention also invented a function for wind turbine power calculations, which is expressed as follows:
      P    TB    ⁢      {                  [                              1            2                    ⁢                      (                          Cx              +              j                        )                    ⁢          a          ⁢                                          ⁢          ρ          ⁢                                    ∑                              i                =                1                            n                        ⁢                                          S                Ci                            ⁢                              d                i                            ⁢                                                                    ω                    i                                    ⁡                                      (                                                                                            k                          i                                                ⁢                        v                                            -                                                                        d                          i                                                ⁢                                                  ω                          i                                                ⁢                        cot                        ⁢                                                                                                  ⁢                                                  α                          i                                                                                      )                                                  2                            ⁢              cos              ⁢                                                          ⁢                              α                i                            ⁢                              sin                2                            ⁢                              α                i                                                    ]            -              P        O              }    ⁢      C    P  
On the condition that: 0<αi<90° & (kiv−diωi cot αi)>0
Where PTB (w) is the wind turbine power; ρ is air density; SC is the area of blade segment i; v (m/s) is the velocity of the wind field; αi(0°) is angle of inclination between the blade and the wind direction; ki is the attenuation coefficient of wind speed before the collision with blade segment i; di(m) is the distance from blade segment i to the turbine shaft; ωi(rad/s) is the turbine angular velocity; a is the number of turbine blades; j is the absorption coefficient of the blades; Cx is the dependent coefficient on the shape of the backside of the blade, which, if flat, will have the highest value of 1.32; Po is the loss in gearbox, generator and friction of the bearings; Cp is the coefficient of power loss when transforming, which will be neglected if the generator is directly connected to the grid.
The invention has also been publicly shared on Youtube in both English and Vietnamese language at: https://Youtu.be/mWxlRlurAp0.
The function for wind turbine power calculations allows for relatively accurate calculations of the power of a wind turbine having blade surface in the form of a thin plate and blade segments short enough to be considered flat, based on factors such as wind speed, rotational speed of the turbine, blade area and the corresponding angle of attack at each position of the blade surface. As a result, it is feasible to design wind turbine blades with optimal technical features for the development of wind power generation.
The patent PCT/VN2015/000007 (Priority date: Jul. 14, 2014, Filling date: Jul. 10, 2015; International publication date: Jan. 21, 2016; WO patent number: 2016/011462) details the manufacture of wind turbine blades with truss-supported structure and twisted surface, which is in line with the viewpoint of a wind turbine blade as an obstruction of the airflow in the process of harnessing wind energy. The wind turbine blade design in patent PCT/VN2015/000007 employs the function for wind turbine power calculations to determine the optimal angle of attack at each position of the blade surface along the blade length corresponding to the rated wind speed and the highest rotational speed of the wind turbine. A set of these attack angles is used to define the overall optimal angle of attack for the turbine blade, which is the angle at the rated wind speed.
The determination of the rated wind speed in patent PCT/VN2015/000007 for defining the angle of attack for the turbine blade is only applicable to variable speed wind turbines. As the wind speed gradually reduces, so will the rotational speed of the turbine, and the appropriate attack angle of the blades will allow the turbine to achieve an output that is comparable to its maximum output thanks to the attack angle determined at that wind speed. For fixed-speed wind turbine, the lower the wind speed, the smaller the turbine output compared to its maximum output thanks to the attack angle determined at that wind speed. Therefore, start-up speed and the lowest speed at which the turbine is forced to stop would be relatively high, and the turbine output at medium and low wind speeds would reduce to a large extent. Based on calculations at the lowest speed at which the turbine is forced to stop, wind turbine output is between 30-40% of the maximum output thanks to the attack angle determined at that wind speed. When the wind speed is just below the rated value, the reduction in the turbine output is negligible. More importantly, since the wind turbine output at that point is already very high, even a further reduction presents no significant impact on the output of the grid.
The control of the attack angle of fixed-speed wind turbine blades in patent PCT/VN2015/000007, however, has not been clear and specific enough. That the attack angle at the blade tip is adjusted to approximately 89° would not be satisfactory in cases of low speed at the blade tip. Besides, the determination of basic parameters for fixed-speed wind turbines has not been detailed to facilitate the design process.